Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Generally, the plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
A typical flexographic printing plate as delivered by its manufacturer is a multilayered article made of, in order, a backing, or support layer; one or more unexposed photocurable layers; a protective layer or slip film; and a cover sheet.
The backing layer lends support to the plate, and is typically a plastic film or sheet, which may be transparent or opaque. For some applications the backing layer can also be a metal such as aluminum or steel.
The photocurable layer(s) can include any of the known photopolymers, monomers, initiators, reactive or non-reactive diluents, fillers, and dyes. The term “photocurable” refers to a solid composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional or relief pattern of cured material. Preferred photocurable materials include an elastomeric compound, an ethylenically unsaturated compound having at least one terminal ethylene group, and a photoinitiator. Exemplary photocurable materials are disclosed in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No. 4,427,759 to Gruetzrnacher, et al., U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each of which is herein incorporated by reference in its entirety. If a second photocurable layer is used, i.e., an overcoat layer, it typically is disposed upon the first layer and is similar in composition.
The photocurable materials generally cross-link (cure) and harden in at least some actinic wavelength region. As used herein, actinic radiation is radiation capable of effecting a chemical change in an exposed moiety. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and infrared wavelength regions. Preferred actinic wavelength regions are from about 250 nm to about 450 nm, more preferably from about 300 nm to about 400 nm, even more preferably from about 320 nm to about 380 nm. One suitable source of actinic radiation is a UV lamp, although other sources are generally known to those skilled in the art.
The slip film is a thin sheet, which protects the photopolymer from dust and increases its ease of handling. In a conventional plate making process, the slip film is transparent to UV light. In this process, the printer peels the cover sheet off the printing plate blank, and places a negative on top of the slip film. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate.
In “digital” plate making processes, a laser is guided by an image stored in an electronic data file, and is used to create an in situ negative on a digital (i.e., laser ablatable) masking layer, which is generally a modified slip film. Portions of the laser ablatable layer are ablated by exposing the masking layer to laser radiation at a selected wavelength and power of the laser.
The laser ablatable layer can be any photoablative masking layer known in the art. Examples of such laser ablatable layers are disclosed for example, in U.S. Pat. No. 5,925,500 to Yang, et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety. The laser ablatable layer generally comprises a radiation absorbing compound and a binder. The radiation absorbing compound is chosen to be sensitive to the wavelength of the laser and is generally selected from dark inorganic pigments, carbon black, and graphite. The binder is generally selected from polyamides, and cellulosic binders, such as hydroxypropyl cellulose. The benefit of using a laser to create the image is that the printer need not rely on the use of negatives and all their supporting equipment, and can rely instead on a scanned and stored image, which can be readily altered for different purposes, thus adding to the printer's convenience and flexibility.
After imaging, the photosensitive printing element is developed to remove the masking layer and the unpolymerized portions of the layer of photocurable material to create a relief image on the surface of the photosensitive printing element. Typical methods of development include washing with various solvents or water, often with a brush. Other possibilities for development include the use of an air knife or heat plus a blotter.
Printing plates with laser ablatable masks can be used to form imaging elements. The flat sheet elements are cut to size and wrapped around a cylindrical form, usually a printing sleeve or the printing cylinder itself, and the edges are fused together or precisely aligned to form a printing element. However, if care is not taken to cover the photocurable surfaces exposed by the cutting process with a material that is opaque to the UV radiation used to expose the plate, a phenomenon called “edge cure” can result.
Edge cure is caused by UV light contacting the cut edges and corners of the plate, which polymerizes the photopolymer and creates an undesirable raised border around the edges of the plate. This border must then be manually cut from the plate, which requires time and can result in damage to the plate, especially if portions of the images are near the plate edge. In addition, removal of the raised border may also leave an undesirable residue on the plate, which must also be removed.
One current process used to prevent edge curing uses a felt tip pen that contains a UV-opaque ink as a means of sealing the edges of such plates. However, this is a slow, tedious process that is only about 90 percent effective at preventing edge curing of the plate.
Another process is described in U.S. Pat. No. 6,326,124 to Alince et al., the subject matter of which is herein incorporated by reference in its entirety. Alince et al. discloses an edge-covering material containing at least one soluble, film-forming polymer, at least one UV absorber, and a solvent or solvent mixture that is applied on the edges of a photocurable printing plate before imagewise exposure of the printing plate to prevent unwanted ridges that result from exposure of printing plate edges. The edge-covering material is applied by brushing or spraying. However, this method is also labor intensive and can be imprecise as the edge-covering material is manually applied to the cut edges.
Thus, there remains a need in the art for improved methods of treating cut surfaces (i.e., edges and corners) of printing plates to prevent the formation of unwanted ridges on the edges of the plate and for a method that can be performed more quickly and accurately than processes described in the prior art.
The inventors have surprisingly discovered that edge curing can be substantially eliminated by using an inkjet print head to print a UV-opaque coating onto the edges of a printing plate after the printing plate has been cut to the desired size and shape. Use of inkjet printing allows for the cut surfaces of the plate to quickly and accurately be coated with a UV-opaque ink. The improved process of the invention is automated and is thus faster, more precise, and more effective than the comparable manual processes of the prior art.